SAS internal IDs must have their passwords stored with them in a SAS metadata repository. These passwords are stored in an encrypted form. OS userids do not normally have passwords stored in SAS metadata, so from that perspective they are more secure.
Perhaps the biggest risk with SAS internal IDs is their passwords are static and there is no mechanism to have them expire automatically so they get changed on a regular basis. So to mitigate this risk you could have a policy of updating them on a regular basis. It is advisable to have these IDs stored in your company's password safe, so they aren't lost.